Oxygen Inhibition Strategies in LED Curing

Experimental Comparison of Various AntiOxygen Inhibition Strategies in LED Curing
Branislav Husár/Institute of Applied Synthetic Chemistry, Vienna University of Technology/Vienna,
Austria
Samuel Clark Ligon§/Institute of Applied Synthetic Chemistry, Vienna University of Technology/Vienna,
Austria
Harald Wutzel/Institute of Applied Synthetic Chemistry, Vienna University of Technology/Vienna,
Austria
Helmuth Hoffmann/Institute of Applied Synthetic Chemistry, Vienna University of Technology/Vienna,
Austria
Robert Liska§/Institute of Applied Synthetic Chemistry, Vienna University of Technology/Vienna, Austria
§
Christian Doppler Laboratory (Photopolymers in digital and restorative dentistry)
Abstract
Photocuring of coatings with LEDs in air suffers from the detrimental effect of molecular oxygen
causing low double bond conversion and poor properties of a cured coating. The objective of this study
is to assess the performance of various anti-oxygen inhibition strategies by comparing double bond
conversions at low light intensities.
1. Introduction
UV curable coatings based on the radical polymerization of acrylates are an increasingly popular
alternative to traditional solvent-borne coatings. However, the problem of the inhibition of radical
polymerization by ground state molecular oxygen has not been solved yet. Incomplete curing leads
generally to diminished mechanical performance and tacky surfaces [1]. In the coatings industry, this
problem has been traditionally mitigated by using a combination of two photoinitiators, whose
absorption characteristics match the emission bands of the mercury lamp. This method is inapplicable
when light emission diodes (LEDs) are employed.
To overcome the associated problems of oxygen inhibition, manufacturers are often reliant on rather
costly nitrogen inerting. Utilization of wax additives, liquid barriers, or lamination are inappropriate for
the SME end users. Another way to reduce oxygen inhibition is to increase curing light intensity but this
option is limited to certain threshold, above which termination overtakes propagation.
Additive based anti-oxygen inhibition strategies can be classified according to the role or roles they
play on the reactions of photoinitiation, radical polymerization, and associated side reactions with
oxygen (Scheme 1). Formation of unreactive peroxyl radical (POO•) can be minimized by reducing the
concentration of oxygen during or prior to initiation. This can be achieved e.g. by addition of sensitizers
in combination with singlet oxygen scavengers or inert gas producing compounds [2, 3]. If oxygen is not
completely removed prior to initiation, all is not lost. Hydrogen donors can react with POO• to form a
reactive radical (D•) capable of propagating polymerization or alternately scavenging an additional
molecule of oxygen. Typical hydrogen donors are amines [4], thiols [5], and ethers [6] but also less
common silanes [7], stannanes [8], hydrogen phosphites [9], or aldehydes [10]. Unreactive peroxyl
radicals (POO•) can also be transformed by reducing agents to a new reactive radical (PO•) that may
propagate polymerization. Typical representatives of this group of reagents are phosphines [11],
phosphites [12], boranes (which are often complexed with amines to improve stability) [13]. Finally,
strategies to regenerate radicals by decomposing peroxide species with photosensitizers are considered
[14]. Strategies based on modification to the monomer formulation may not be accurately depicted by
Scheme 1 explaining the role of additives. Such strategies include acrylates bearing aforementioned
reactive functional groups [6], multifunctional acrylates [15], acrylated dendrimers [16], N-vinyl amides
[17], donor/acceptor type monomers [18], and hybrid radical/cationic systems [19].
(k)
M
(e)
RA
In oxygen-free atmosphere
P
P M
DH
(b)
P
O2
M
P
O
P
PI
T
P
O RA
+
P O
O
OH
+
D
O
O
P
(i)
P O
R
(j)
O2
(a)
(h)
RH
P
O
(i)
OH
P O
+
H O
+
O2
R
sensitizer
stable
(d)
1O
RA
(g)
O
(c)
PI
O
(f)
polymer
h
O
SOS
2
SOSO2
Scheme 1. Mechanistic explanation of oxygen inhibition and strategies to mitigate it: (a) quenching of
excited state of photoinitiator, (b) formation of unreactive peroxyl radicals from initiating or propagating
radical, (c) initiation stage strategies (i.e., inerting, lamination, light source, molecular inerting, and
photoinitiators), (d) singlet oxygen scavengers, (e) reducing agents, (f) hydrogen donors, (g) termination
by radical−radical recombination, (h) hydrogen abstraction, (i) peroxide decomposition, (j) scavenging
of a molecule of oxygen, and (k) reinitiation of polymerization.
In this study, we attempted to assess the effectiveness of various anti-oxygen inhibition additives by
determination of the DBC of the cured additive containing formulation and comparing to the DBC of the
cured base (additive-free) formulation. FTIR is a precise and quantitative method for assessing cure
performance, but it is limited at high conversions where DBC may not correlate well with the
mechanical properties of the cured coating [20].
2. Experimental
The base formulation consisted of a 1:1 w/w mixture of polyether urethane diacrylated (Bomar BR344) and DPGDA with 2 wt% of Speedcure BDMB as a photoinitiator. Various additives were then
added to this base formulation in amounts expressed in molar equivalents to BDMB. Freshly prepared
films (6 µm) on polyethylene foil were placed into the FTIR spectrometer and irradiated for 10 s with
LED lamps. Sample films without ITX were irradiated the 365 nm LED lamp (120 mW/cm2 at the
surface of the film). For samples prepared with ITX, the film was irradiated simultaneously with 365
and 400 nm LED lamps (2 × 60 mW/cm2 at the surface of the film). All measurements were performed
at least 5 times to ensure the reproducibility of the results. Finally, double bond conversion (DBC) was
determined from the decrease of the 1620 and 1637 cm−1 bands using the carbonyl band at 1726 cm−1 as
a reference.
3. Results and Discussion
The results are summarized in Figure 1 and 2. Figure 1 shows the results acquired after curing at 365
nm in air. The full line displays the DBC (27%) achieved from the curing of the base formulation under
air without any additive. The dashed line shows the DBC (79%) obtained from the same base
formulation cured under laminated conditions. Data presented in Figure 2 were obtained after dualwavelength curing at 365/400 nm in air. The addition of ITX (0.14 wt%) increases DBC of the base
formulation from 27% (365 nm) to 42% (365/400 nm) at the same total light intensity and dose. The full
line displays the DBC (42%) achieved after curing in air, while the dashed line shows the DBC (84%)
under laminated conditions.
100
90
80
70
60
50
40
30
20
10
0
Figure 1. Double bond conversions of formulations containing various additives cured at 365 nm in air.
100
90
80
70
60
50
40
30
20
10
0
Figure 2. Double bond conversions of formulations containing ITX as sensitizer and various additives
cured simultaneously at 365/400 nm in air.
3.1 Hydrogen Donors
A general scheme for the mechanism of hydrogen donation is shown in Scheme 2. The hydrogen
donor donates a hydrogen atom, which caps the already formed peroxyl radical. The formed donor
radical (D•) reinitiates polymerization or scavenge a molecule of O2.
O
R
O
O
R
O
O
OH
propagation
DH
O
R'
C
H
D
O
R'
D
O2
oxygen scavenging
D
O
O
Scheme 2. Radical reinitiation via hydrogen donation.
In this study, a series of hydrogen donors (amines, thiols, silane, hydrogen phosphite, stannane,
aldehyde) has been tested (Scheme 3).
O
N
Me
HO
N
DABCO
N
OH
Si
TTMSS
Ph
N
Ph
HS
O
O
O
O
Bz3N
MDEA
HS
O
O
O
O
O
O
SH
TMPMP
SH
O
HS
O
PETMP
SH
O
Bu
Si
Si Si H
SH
Ph
O
H35C18
O
P
H
Bu
O
D253
C18H35
O
Sn H
Bu
Bu3SnH
MeO
PAA
Scheme 3. Tested hydrogen donors.
N-methyl diethanolamine (MDEA) and tribenzyl amine (Bz3N) were found more effective than 1,4diazabicyclo[2.2.2]octane (DABCO). Addition of MDEA and Bz3N to the formulation increased the
DBC from 27% to 38%, while with DABCO only 32% DBC was reached. MDEA is thus preferred
since Bz3N is a solid and has a much high molecular weight. In the presence of ITX, MDEA gives
similar enhancement with DBC increasing from 42% to 51%. Bz3N by comparison seems to be less
effective in the presence of ITX. Odor, volatility, water solubility, and reactivity with atmospheric acids
are issues with low molecular amines. Therefore, amine acrylate monomers are a good alternative.
Thiols are effective hydrogen donors and oxygen scavengers. In this study, a trithiol (TMPMP) and
a tetrathiol (PETMP) were used. These multifunctional thiols copolymerize and therefore do not leach
from the final coating. In comparison to amines tested at equal molar concentrations, TMPMP and
PETMP provided superior results (46% and 47% DBC respectively). The main problems associated with
thiols are their bad odor and their low storage stability. The problem of odor can be overcome by using
polymeric thiols lacking cleavable groups. Storage stability can be improved with use of appropriate
radical inhibitors and phosphonic acid coadditives [21].
Hydrogen phosphites HP(O)(OR)2 have been scarcely reported as anti-oxygen inhibition additives
[9]. In this study, dioleyl hydrogen phosphite (D253) provided an enhancement in cure comparable to
MDEA (38% DBC). Thus, no advantage over amines can be seen.
Silanes were reported to be effective hydrogen donors [8]. Tris(trimethylsilyl)silane (TTMSS) has
Si−H bond energy (79.8 kcal·mol−1) considerably lower than the C−H bond energy in MDEA (87.1
kcal·mol−1). Despite that, TTMSS gives an improvement comparable to MDEA. TTMSS is air and water
sensitive, heavier, and more expensive. Amines seem to be the better option.
Tributyl stannane (Bu3SnH) contains a very weak Sn−H bond (73.8 kcal·mol−1) and donates H-atom
to peroxyl radical about 10 times faster than TTMSS [8]. Although Bu3SnH partially hydrolyzed in nondried base formulation, the curing reached as much as 54% DBC. Despite a superior improvement,
Bu3SnH is not of industrial interest due to high toxicity and high moisture sensitivity.
Aldehydes have been described as effective coinitiators in Type II initiating systems [10]. In this
study, aldehydes were tested as hydrogen donors in the presence of a Type I initiator. Addition of PAA
did not provide a statistically significant improvement to the base formulation.
3.2 Reducing Agents
Reducing agent are capable of reducing peroxyl radicals. In the process, new radicals are formed
that may propagate reaction. Various phosphines, phosphites, sulphite, and borane were tested (Figure
3).
C8H17
Ph
P
Ph
H17C8
Ph
PPh3
ALK
ALK
O
P
O
Ph
C8H17
O
P
Ph
O
O
Ph
Ph
O
P
Ph
O
O
O
O
O
P
P
O
O
O
Ph
Ph
O
O
P
D11
ALK
ALK
P
O
Ph
O
O
O
P
O
Ph
O
Ph
7
P(OPh)3
TOP
O
Ph
O
H27C13
O
P
D12
O
C13H27
O
O
C13H27
O P
H37C18
D49
D613
O
S
O
P O
O
O
BOTDBU
O
NH
BH3
C18H37
ETS
Me2NH.BH3
Figure 3. Tested reducing agents.
Phosphines and phosphites are very effective antioxidants. In photopolymerization, they may be
oxidized by peroxyl radicals. In the process, phosphine/phosphite oxides and alkoxyl radicals are formed
that reinitiate the polymerization (Scheme 4).
Ar
Ar
P
R
Ar
O
OOR
O
Ar
P Ar
Ar
O
Ar
P Ar
+
R O
Ar
Scheme 4. Radical reinitiation via phosphine oxidation.
Triphenyl phosphine (PPh3) is one of the most effective additives in this study. Under the testing
conditions, 3 eq. gives a curing performance beyond that of the laminated sample (91% DBC) with no
final surface tack. A freshly prepared formulation containing 1 eq. of PPh3 gives 60% DBC, which
decreased to 52% after 4 weeks of storage at room temperature in a sealed flask under air. Some
turbidity and increased viscosity was observed. Moreover, when stored in an unsealed flask in air,
approximately 10% of the initial acrylate groups in the formulation had vanished although the
formulation did not gel. An aliphatic trioctyl phosphine (TOP) proved to be much less effective than
PPh3. DBC reached only 38%.
Phosphites proved to be the second most effective type of additive giving impressive results ranging
from 50 to 64% DBC. The aromatic triphenyl phosphite (P(OPh)3) was the least effective (50% DBC)
that can be attributed to the reaction with alkoxyl radicals to form unreactive phenoxyl radicals. On the
other hand, the aliphatic tris(tridecyl) phosphite (D49) gave the highest DBC in this study (64%).
Phosphites are generally liquids and easy to dissolve in the formulation. Only BOTDBU is a solid and
rapidly precipitates from the formulation. Price, viscosity, and color appear to be sufficiently low for
wood coating applications. However, phosphites exhibit storage stability problems in some ways
comparable to PPh3. A slightly higher viscosity and turbidity was observed upon storage in a sealed
flask under air for 4 weeks. In the case of the oligomeric D12, the DBC dropped from 56% to 44% after
4 weeks of storage. It is worth noting that phosphites also have a distinct phenolic odor independent of
molecular weight, which could be detected even in the least volatile oligomeric D12.
Sulfites are known antioxidants but have not been reported to date as anti-oxygen inhibition
additives for photopolymerization. As a representative of this class of molecules, we chose ethylene
sulfite (ETS). Based on these results, the mediocre performance (36% DBC) and irritancy make them
less appealing for industrial application.
Boranes in the presence of oxygen act as radical initiators. A useful strategy for stabilizing boranes
during storage and yet to keep it available for intended oxygen inhibition is to complex it with an amine
[13]. In the presence of a Type II photoinitiator, H-abstraction from the amine takes place. Released
borane (BH3) provides reactive radicals after the reaction with dissolved oxygen. We attempted to use
the dimethylamine-borane complex (Me2NH·BH3) with a Type I photoinitiator (BDMB) only. The
effect was comparable to that of amines (38% DBC). Surprisingly, in the presence of ITX (0.1 eq.) the
increase in DBC was negligible. Under these conditions the effect of boranes is questionable and may
not justify its use in light of issues such as cost, stability, and solubility.
3.3 N-Vinyl Amides
N-vinyl pyrrolidone (NVP) has often been added to UV curable resins to reduce viscosity and
improve the curing in air. It has been suggested that (i) NVP acts as an oxygen scavenger similar to
amines or (ii) NVP forms an exciplex with oxygen to produce reactive radicals or (iii) NVP forms a
donor-acceptor complex with acrylate [17]. NVP gives 40% DBC while its non-vinyl analogue NMP
gives just 33% DBC after curing in air. Although NVP seems to be an effective additive, similar results
can be obtained with low molecular additives of similar molecular weight. Butyl acrylate gives 36%
DBC and butyl acetate gives 34% DBC under the same curing conditions. These results can be
explained by increased mobility of monomers. One must be also aware of increased hydrophilicity of the
cured photopolymer, which is generally undesirable in wood coating applications.
O
O
N Me
N
NVP
NMP
Figure 4. N-vinyl pyrrolidone (NVP) and N-methyl pyrrolidone (NMP).
3.4 Molecular Inerting
Some radical photoinitiators (Figure 5) undergo decomposition to provide initiating radicals and a
molecule of CO2. This gas has a potential to suppress oxygen inhibition by replacing oxygen in the
formulation. These compounds are used in combination with a photosensitizer such as ITX and
decompose according to the following scheme (Scheme 5).
O
Ph
H
N
COOH
NPG
Ph
N
COOH
NMNPG
Ph
S
COOH
PTAA
Ph
N
O
POE
O
O
O
Me
Ph
Ph
N
O
PDO
O
+
S
ITX
Figure 5. Tested CO2 producers and sensitizer (ITX).
a)
Ph
b)
Ph
H
N
h
COOH
Ph
ITX
H
+
N
COOH
Ph
O
N
CH2
+
CO2
O
h
O
H
N
Ph
N
Ph
ITX
+
Ph
Ph
O
N
+
Ph
+
CO2
Scheme 5. Photo-induced gas generation from a) N-phenyl glycine (NPG) and b) O-benzoyloxime
benzaldehyde (POE).
NPG, NMNPG and PTAA showed all negligible improvements in DBC after curing. ITX would be
ideally used in equimolar ratio to these compounds, however it is not possible due to nonphotobleachable nature of ITX.
Oxime esters (POE, PDO) seem to be a promising strategy at first, with improvement in DBC from
42 to 49% at low additive content (0.3 eq.). POE and PDO are also photoinitiators absorbing at 365 nm
similarly to BDMB. However, addition of 0.3 eq. BDMB to base formulation with 0.1 eq. ITX improves
curing to 55% DBC.
3.5 Singlet Oxygen Scavengers
Singlet oxygen (1O2) does not react with radicals during the polymerization process. 1O2 can be
generated from 3O2 by a suitable photosensitizer, e.g. ITX. Due to the short life-time of 1O2, it is
essential to trap it before it can relax back to 3O2. This is accomplished by using singlet oxygen
scavengers, which undergo [4+2] cycloaddition reactions (Scheme 6).
3O
3O
2
ITX
1O
Ph
O
Bu
2
Ph
Ph
DPF
2
ITX
O O
O
1O
2
Bu
O O
Ph
Bu
Bu
DBA
Scheme 6. Singlet oxygen scavenging via [4+2] cycloaddition with diphenyl furan (DPF) and dibutyl
anthracene (DBA).
In previous investigations, 2,5-diphenyl furan (DPF) gave very promising results in a formulation
containing camphorquinone as the initiator reaching the same DBC in air as the formulation without
DPF cured under nitrogen [2]. However, in this study the use of DPF without or with ITX did not
increase double bond conversion. The usage of 9,10-dibutyl anthracene (DBA) in combination with ITX
improved DBC from 42% to 55%. Important to clear coat applications, DBA photobleaches upon curing
and the photoproduct is a stable molecule. DBA showed good storage stability in the formulation within
30 days.
Conclusions
To provide an alternative to nitrogen inerting, a variety of anti-oxygen inhibition additive were
evaluated. Reducing agents are the most effective additives in improving final cure, however
formulations are in almost all cases not storage stable. Hydrogen donors were found to improve cure up
to a certain extent but due to different limitations only amines, borane-amine complexes and thiols seem
applicable industrially. N-vinyl amides are suitable for applications where increased hydrophilicity and
reduction in viscosity can be accepted. CO2 producers provided only little improvement. The use of
singlet oxygen scavengers seems to be a promising approach.
Acknowledgements
The authors wish to thank the European Commission for financial support of the FP7 SME project
“Energy efficient UV LED curing without inerting” (Grant No. 262514). B.H. and R.L. gratefully
acknowledge the support by the European Commission in the framework of Marie Curie Intra-European
Fellowships through the project VINDOBONA (Grant No. 297895). Additional thanks for kind donation
of chemical additives go to Lambson Ltd. (photoinitiators), Bomar Specialties (oligomer), Bruno Bock
Chemische Fabrik (thiols), and Dover Chemical Corporation (phosphites).
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